This invention relates to digital communication receivers that estimate or track multi-rate fading channels and, more specifically, to a method for symbol-spaced estimation and/or tracking of a fractionally-spaced channel.
Digital cellular systems require equalization to handle intersymbol interference (ISI) from time dispersion or delay spread in the channel. Typically, a non-linear equalizer (e.g., maximum likelihood sequence estimator (MLSE)) is used for such channels. An estimate of the radio channel is required for this equalization. Furthermore, if the channel changes fast enough, then the equalizer must continuously track the variations in this channel. Our invention provides a novel method for estimation or tracking of such radio channels. This method is specially suited for those situations where the medium rays are spaced closer than one symboling interval.
A radio channel can be effectively broken into three parts. These are the transmit filter, which is known, the channel medium, which is unknown, and the receiver filter, which is known. The inputs to the radio channel are typically symbols from a discrete alphabet, such as +1 and xe2x88x921, with a sampling rate equal to the baud rate. The transmit filter, the channel medium, and the receiver filter are typically modeled at a higher sampling rate, for example eight times the symbol rate. The equalizer uses a symbol-spaced model of the radio channel that relates the transmitted symbols to the output of the receive filter at the sampling rate of one sample per symbol period (this application refers to this symbol-spaced channel as the effective baseband channel). The reference xe2x80x9cDigital Communicationxe2x80x9d (Lee/Messerschmitt, Kulwer Academic Publishers, 1994) presents a method for directly estimating the effective baseband channel. Similarly, the reference xe2x80x9cLicentiate Thesis, Lars Lindbum, Upsala University 1992, ISSN 03468887) presents a method for directly tracking the effective baseband channel. However, the present invention provides an improvement over these methods when the underlying rays of the channel medium are spaced closer than one symbol period. A. Khayrallah, R. Ramesh, G. Bottomley, and D. Koilpillai, xe2x80x9cImproved Channel Estimation with Side Informationxe2x80x9d xe2x80x9cVTC-97, Vol.2, pp.1049-1051, presents a method for estimating fading mobile radio channels were the effective baseband channel is modeled as a convolution of a known pulse and an unknown medium. The authors only estimate the medium part of the channel explicitly, and reconstruct the effective baseband channel as a convolution of this estimated medium and the known pulse. However, as shown in the present application, the effective baseband channel cannot always be expressed as the convolution of a known pulse and an unknown medium. Specifically, the method of Khayrallah is not applicable when the channel medium has rays that are placed closer than one symbol-period (i.e., fractionally-spaced medium).
In accordance with the invention there is provided a method for symbol-space estimation or tracking of fractionally-spaced radio channels.
Broadly, there is disclosed herein the method of estimating time varying radio channels comprising the steps of receiving signals transmitted over the channels and converting the signals to digital samples for processing; determining positions of fading rays of the channel medium; estimating impulse response of a channel medium for each of the estimated positions; and calculating estimates of the time varying radio channels using the digital samples and the estimated impulse responses.
It is a feature of the invention that the fading rays are spaced closer than a transmit symbol period.
It is another feature of the invention that the radio channel estimates are provided for a GSM equalizer.
It is a further feature of the invention that the radio channel estimates are provided for a code-division multiple access equalizer.
There is disclosed in accordance with another aspect of the invention a channel estimator adapted to track time varying radio channels including means for receiving signals transmitted over the channels, and converting the signals to digital samples for processing. Means are provided for determining positions of fading rays of the channel medium. Means estimate impulse response of a channel medium for each of the estimated positions. Means are provided for calculating estimates of the time-varying radio channel using the digital samples and the estimated impulse responses of the medium.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.